Clever technology decodes more information from single photons

Feb 12, 2014

This NIST device, 1.5 by 3 centimeters in outer dimensions, is a prototype receiver for laser communications enabling much higher data rates than conventional systems. Superconducting detectors in the center of the small square chip register the timing and position of single particles of light. Credit: NIST

It's not quite Star Trek communications—yet. But long-distance communications in space may be easier now that researchers at the National Institute of Standards and Technology (NIST) and Jet Propulsion Laboratory (JPL) have designed a clever detector array that can extract more information than usual from single particles of light.

Described in a new paper, the NIST/JPL array-on-a-chip easily identifies the position of the exact detector in a multi-detector system that absorbs an incoming infrared light particle, or photon. That's the norm for digital photography cameras, of course, but a significant improvement in these astonishingly sensitive detectors that can register a single photon. The new device also records the signal timing, as these particular single-photon detectors have always done.

The technology could be useful in optical communications in space. Lasers can transmit only very low light levels across vast distances, so signals need to contain as much information as possible.

One solution is "pulse position modulation" in which a photon is transmitted at different times and positions to encode more than the usual one bit of information. If a light source transmitted photons slightly to the left/right and up/down, for instance, then the new NIST/JPL detector array circuit could decipher the two bits of information encoded in the spatial position of the photon. Additional bits of information could be encoded by using the arrival time of the photon.

The same NIST/JPL collaboration recently produced detector arrays for the first demonstration of two-way laser communications outside Earth's orbit using the timing version of pulse position modulation. The new NIST/JPL paper shows how to make an even larger array of detectors for future communications systems.

The new technology uses superconducting nanowire single-photon detectors. The current design can count tens of millions of photons per second but the researchers say it could be scaled up to a system capable of counting of nearly a billion photons per second with low dark (false) counts. The key innovation enabling the latest device was NIST's 2011 introduction of a new detector material, tungsten-silicide, which boosted efficiency, the ability to generate an electrical signal for each arriving photon. Detector efficiency now exceeds 90 percent. Other materials are less efficient and would be more difficult to incorporate into complex circuits.

The detectors superconduct at cryogenic temperatures (about minus 270 °C or minus 454 °F), and cooling needs set a limit on wiring complexity. The NIST/JPL scheme requires only twice as many wires (2N) as the number of detectors on one side of a square array (N x N), greatly reducing cooling loads compared to a one-wire-per-detector approach while maintaining high timing accuracy. NIST researchers demonstrated the scheme for a four-detector array with four wires and are now working on a 64-detector array with 16 wires.

In the circuit, each detector is located in a specific column and row of the square array. Each detector acts like an electrical switch. When the detector is in the superconducting state, the switch is closed and the current is equally distributed among all detectors in that column. When a detector absorbs a photon, the switch opens, temporarily diverting the current to an amplifier for the affected column while reducing the signal through the affected row. As a result, the circuit generates a voltage spike in the column readout and a voltage dip in the row readout. The active detector is at the intersection of the active column and row.

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User comments : 20

Wow! The mind boggles. Even more information can be achieved by finding ways to electronically polarize a single photon, or modulate a single photon. Unless I'm reading this wrong, the array would simply serve as a fail-safe redundancy measure if they can get those single photons to entangle, and we would have instantaneous communication over astronomical distances.

Wow! The mind boggles. Even more information can be achieved by finding ways to electronically polarize a single photon, or modulate a single photon. Unless I'm reading this wrong, the array would simply serve as a fail-safe redundancy measure if they can get those single photons to entangle, and we would have instantaneous communication over astronomical distances.

Jeezus, why did I put myself in this position (I am just talking to myself here). I will do my best, and I hope you will do your part to complete your understanding. For the second part of the last sentence you will need some basics of quantum field and relativity theories. By the way one does not need to be a genius to understand them, but you have to be willing to leave the comfort of your usual intuitions. For now, I will try to stay as much on usual intuitions and perceptions as possible. What it means is that I will have to use the photon-ball analogy, and this is an incorrect way to describe it.

A photon is a quantum object that has an axial spin, and a direction. It also has a fix energetic value as described by the photoelectric effect E=hf: The energy of a photon is equal to its frequency multiplied by the Plank constant. The direction is what it is; there is no explanation to add, if we leave relativity on the side. The spin of a photon is the angular momentum that gives rise to the electromagnetic phenomenon that you perceive as light when the electromagnetic wavelength is in the visible spectrum. You can think of it as a ball that is spinning on its axis as it travels in space, except that it is also a relativistic object that travels at light speed; hence it has no mass and dimensions (It is impossible for objects that have any mass or dimensions to travel at light speed).

A photon is a real entity (I mean that they are not only theoretical objects). It is possible to detect and measure the energy of a single photon, it is possible to trap one in a resonant cavity, but one can never hold a photon (It only exists at the speed of light).

There is still much to say as I did not talk about the spin components, and entanglement. A proper basic description would probably take as much time as I have took so far. But I have already spent a good two hour to choose the right words to help you have a basic understanding of photons. English, for me, is not such a foreign language, but add some difficulties.

The usual mistake that people do when they think about a photon, is thinking of it like a short electromagnetic pulse; it is not. As for information exchange between entangle particles not possible either. What is entanglement is still under scientific investigation so any explanations are highly hypothetical. Anyway, I will give you a youtube link, if you have an hour to spare, it is very well explained and in simple words; it is the best hypothetic explanation that I have heard so far.

This article is not about modulation, but about encoding information into spin of photon, not just frequency.

If quantum entities cannot be modulated then radio waves cannot be modulated

The electromagnetic waves spread with antenae (i.e. systems of many electrons entangled) do represent a system of many photons entangled. Such a system can be modulated with energy, which is the 1/N fraction of energy level (N denotes the number of all photons entangled in a space-time interval used for modulation). This explains, why EM signal can be modulated so slightly. Actually, in AWT it has no meaning to talk about individual bosons in vacuum for EM waves of wavelength larger than CMBR wavelength, as the photons do behave like the unstable tachyons there: they're getting scattered with CMBR fluctuations fast.

@BaudrunnerHere I have found a freely available paper on arXiv that gives a proper mathematical description of photons. I am not suggesting you to learn that, as the mathematical language used is highly strenuous, and take many years to master. But it will give you an appreciation of what it is to describe photons.

Be careful, in the second sentence of the introduction of this paper the author writes 'the photon is the quantum of the electromagnetic field'; it is true, but it is a shortcut that is misleading and can bring you back to view a photon as an electromagnetic pulse. So I will repeat myself, the electromagnetic phenomenon is just a manifestation of photons.

This article is not about modulation, but about encoding information into spin of photon, not just frequency.

Well, yes it is, actually, because the encoding of information into spin of a photon is effectively modulating it.

Creed, the photons that represent what you see actually originate within your eye, where the "information" is absorbed and converted into the bio-electrical signals that travel via the optic nerve to your occipital cortex. The information is frequency and intensity, that is all.

When Einstein said that nothing travels faster than light, he was really (and perhaps unwittingly) referring to the rate of the atomic interaction of adjacent EM wave carriers in the medium. A denser medium naturally means more atoms for a given distance therefore light appears to travel slower through, say, water, or glass.

It is not wrong to think of a photon as a short electromagnetic pulse. In fact, Planck was the first to coin the term "quanta" to describe a single pulse of a wave ejected from a black body cavity. Einstein was the first to coin the term "photon" to describe the quanta of electro-magnetic energy required to evict a single electron from a sodium atom in a Bell jar by radiating a sodium block with ultraviolet light, which deflected the needle in an ammeter. He won the Nobel prize for his description of this "photo-electric effect", and not for his relativity theories, as most people believe.

A single photon is not a standard unit of energy. The amount of energy that a single photon possesses is a function of frequency, intensity, and source.

On the subject of entanglement, it is not so difficult to understand if you but replay a simple thought experiment, that of a long straight pole that extends from here to Mars. If you nudge the pole forward, the effect would be felt immediately on Mars. Ignore for a moment all of the postulated explanations that challenge the supposed effect of nudging that pole, they exist only to meet the requirement that nothing can travel faster than light. Information can, however, travel faster than light, and the effect on Mars is receipt of the information that the pole has been nudged. Entanglement consists therefore of nothing more than managing a single photon wave, whereby changing the spin characteristics of the outer electron shell of the emitting source - ie. polarizing the wave - effects an immediate change at the receiving end of the wave.

@bd - i like your enthusiasm but your arbitrarily-long conrod would also need to be arbitrarily rigid (giggity), whereas in reality you'd expect the translation to propagate at finite speed, much as a vibration along it.

I admit this is counter-intuitive to accept, but in the same way as the 'dropped slinky delay' effect, where the lower end of the spring seems to remain suspended mid-air.. OK crap analogy, but information can't travel faster than light or causality violations could result.

The NIST/JPL work is promising though. In principle, i don't see why additional spatial and temporal variables couldn't be included, such as amplitude, wavelength and carrier FM (ie. the relationships between multiple freqs, as well as their discrete modulations like polarisation and interval)... it might thus be possible to squeeze 5 or 6 bits into a pair of photons..

@MrV - Absolute rigidity of the rod is assumed, it is a thought experiment after all. And simply pushing the rod forward cannot be compared to running a vibration along it. I know this is getting needlessly intricate, but small details matter.

As to causality violations, God is himself a causality violation, if you believe in him. Somehow, apparently, he existed before anything that he created existed in order for him to be extant. So, I guess creationists have to believe in time travel.. (I'm not a creationist - just saying ;)

Well it'd also need to be arbitrarily thin - or else its inertial mass will require inordinate energy to get it moving noticeably.

I played with a similar thought experiment quite recently - same deal but with an arbitrarily long scissorjack. It's an interesting implement because the struts go from near-horizontal, when fully contracted, to near-vertical when fully extended, in such a way that the acceleration of the leading scissor section is the additive sum of all previous sections - with say six or more sections, the leading edge shoots forward with surprising speed. Hence i was struck by the thought that with sufficient no. of sufficiently-long scissor sections, the leading pair would quickly exceed C.

I concluded however that the beguiling scissoring action, giggity, was just a red herring, and ultimately all that matters is the conventional mass displacement / energy constraints, regardless of the mechanism applied....

The formation of photons from light waves isn't any mystery in AWT and it's just an analogy of formation of solitons at the water surface. It all boils to the fact, that the space-time isn't smooth homogeneous stuff, but it's filled with density fluctuations of so-called Higgs field. Every elastic deformation of it therefore brings additional density fluctuations into reality in similar way, like the specific surface area of undulating surface increases together with amount of density fluctuations exposed to it. Therefore the undulating place behaves like the weak gravitational lens and it focus the traveling wave into itself. If the space-time would be completely smooth, it would spread the light waves pretty much in the way, which the Maxwell's theory predict: like the spherical waves.

The trick with entanglement of photons is, that the two photons may share the single gravitational lens, which can separate and connect with speed of longitudinal waves of vacuum.

These longitudinal waves are propagating in extradimensions and they're actually required for explanation of density fluctuations of Higgs field. They manifest itself with CMBR noise, which correspond the Brownian noise generated with underwater longitudinal waves at the water surface. So that the same effect which is responsible for photon formation is responsible for their entanglement. When two photons get entangled, they're synchronized in their phase. The density wave of the vacuum spreads independently, so it may disconnect both photons and reconnect them again somewhere else.

The entanglement of massive particles is easier to imagine, as such particles stay at rest. You may imagine it like the formation of standing wave on the surface of large droplet inside of lava lamp. When such droplet gets separated, it's surface ripples continue in bouncing along surface of resulting droplets in phase. When these droplets will merge again, their surface ripples will merge seamlessly too.

Both undulating halves therefore remember their origin, which is defined with frequency, amplitude and phase of surface waves. If one of droplets from entangled pair will touch some other droplet (an "observer"), it will get entangled with it instead. The amplitude, phase or frequency of its surface ripples will not match the second half of original droplet anymore: the "observation" of one member of entangled pair will thus ruin the entanglement for the second half immediately - no matter where this second half actually resides.

The process of spooky entanglement and decoherence of entangled pairs across whole universe could be therefore explained easily with internal memory of standing waves, which are spreading in extradimensions along/across all particles (across wake wave of vacuum, created by their motion being more specific). The entangled pair therefore behaves like the pair of boats at the surface of river with their wake waves synchronized in both frequency both phase.

The main difference for photons is, their internal undulations participate on their wake wave which surrounds them in synchrony. When two photons collide, this synchrony could be broken and such photons would collapse into particle-antiparticle pair - but this is another story. What is important to remember, every photon (i.e. soliton by itself) is surrounded with its wake wave, which is of different density and which is responsible for mutual interference of photons in double slit experiments. The photons solitons itself remain independent on their de Broglie wake waves and they do manifest itself like the spots at the target. The energy density separation is the more pronounced, the more the photons are energetic. Only for microwave photons of wavelength of CMBR the undulating interior of photons becomes indistinguishable from it wake wave. The photons of even larger wavelength have less dense core in their belly and they tend to explode like tachyons of negative rest mass.